In recent years calcium carbonate has found a wide array of uses across many fields. For example, calcium carbonate is one of the most widely used minerals in the paper, plastic, paint and coating industries both as a filler and, due to its white color, as a coating pigment. In the paper industry calcium carbonate is valued for its high brightness, opacity and gloss and is commonly used as a filler to make bright opaque paper. In addition, calcium carbonate is frequently used as an extender in paints and is also used as a filler in adhesives, sealants and plastics. High grade calcium carbonate has also found uses in formulations of pharmaceuticals.
Calcium carbonate is known to exist as natural occurring minerals as well as a synthetically produced products.
“Ground natural calcium carbonate (GNCC)” in the meaning of the present invention is a calcium carbonate obtained from natural sources including marble, chalk or limestone or dolomite. Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate. The other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470° C., and vaterite is even less stable. Ground calcium carbonate is processed through a treatment such as grinding, screening and/or fractionizing by wet and/or dry, for example, by a cyclone. It is known to the skilled person that ground calcium carbonate can inherently contain a defined concentration of magnesium, such as it is the case for dolomitic limestone.
“Precipitated calcium carbonate (PCC)” in the meaning of the present invention is a synthesized material, generally obtained by precipitation following the reaction of carbon dioxide and lime in an aqueous environment or by precipitation of a calcium and carbonate source in water or by precipitation of calcium and carbonate ions, for example CaCl2 and Na2CO3, out of solution. Precipitated calcium carbonate exists in three primary crystalline forms: calcite, aragonite and vaterite, and there are many different polymorphs (crystal habits) for each of these crystalline forms. Calcite has a trigonal structure with typical crystal habits such as scalenohedral (S-PCC), rhombohedral (R-PCC), hexagonal prismatic, pinacoidal, colloidal (C-PCC), cubic, and prismatic (P-PCC). Aragonite is an orthorhombic structure with typical crystal habits of twinned hexagonal prismatic crystals, as well as a diverse assortment of thin elongated prismatic, curved bladed, steep pyramidal, chisel shaped crystals, branching tree, and coral or worm-like forms.
Among these forms, the scalenohedral form of calcite is particularly desirable for use as a bulking pigment in the paper industry because it is relatively inexpensive to produce and it has desirable light scattering properties.
Generally, one way to produce calcium carbonate commercially is by calcining crude limestone to obtain quicklime. Water is then added to yield an aqueous suspension of calcium hydroxide (“milk of lime”), and carbon dioxide is reintroduced into this slurry to precipitate the calcium carbonate. The product of this process is known as precipitated calcium carbonate (“PCC”). The resulting aqueous suspension, or slurry, of calcium carbonate may be used as is or further processed (e.g., dewatered, grinded, etc.) to form a dry product. The precipitation reaction is capable of producing each of the three polymorphs (calcite, aragonite and vaterite) depending on the exact reaction conditions used.
Prior art processes for producing scalenohedral PCC product typically rely on the use of additives such as monosaccharides (e.g, simple sugars such as fructose, glucose), disaccharides (e.g., sucrose, maltose, lactose), polysaccharides (e.g, starch, cellulose, glycogen), triethanolamine, mannitol, diethanolamine, bicine, morpholine, tri-isopropanolamine, N-ethyl diethanolamine, N,N-diethylethanolamine, sodium boroheptonate, or reagents including a polyhydric alcohol or a polyhydric phenol, during the slaking of the quick lime or prior to carbonation (see, e.g., U.S. Pat. Nos. 6,294,143, 5,232,678 and 5,558,850).
Conventional processes for preparing scalenohedral PCC also typically cool the slaked lime before carbonation (see, e.g., U.S. Pat. Nos. 3,320,026 and 6,251,356).
In addition, conventional processes for preparing scalenohedral PCC utilize agitation during carbonation (see, e.g., U.S. Pat. Nos. 3,320,026, 5,232,678, 5,342,600, 5,558,850 and 6,251,356).
In the manufacture of paper, and particularly woodfree paper, there is a desirability of increasing the filler content to achieve higher bulk, and at the same time, increasing the stiffness of the produced/obtained paper. However, one of the downsides of conventional scalenohedral PCC is that it may not be as strong as required in the manufacture of paper, and particularly uncoated woodfree paper. Accordingly, there exists a need for a low cost process for producing precipitated PCC in the scalenohedral form that is stronger than conventional scalenohedral PCC, that permits an increase in the filler content and density without sacrificing stiffness or bulk of the produced paper.